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Four antibacterial compounds were isolated from leaves of guava ( Psidium guajava L.), and the structures of these compounds were established on the basis of chemical and spectroscopic evidence. Two new ‰avo-noid glycosides, morin-3-O-a-L-lyxopyranoside and morin-3-O-a-L-arabopyranoside, and two known ‰avo-noids, guaijavarin and quercetin, were identiˆed. The minimum inhibition concentration of morin-3-O-a-Llyxopyranoside and morin-3-O-a-L-arabopyranoside was 200 mg W ml for each against Salmonella enteritidis, and 250 mg W ml and 300 mg W ml against Bacillus cereus, respectively.

The antibacterial activities of flavonoids were found by the paper disk method to be enhanced by combining or mixing them. The combinations of quercetin and quercitrin, quercetin and morin, and quercetin and rutin were much more active than either flavonoid alone. Although rutin did not show activity in itself, the antibacterial activities of quercetin and morin were enhanced in the presence of rutin. The antibacterial activities of flavonoids, in combination with morin and rutin, were evaluated, based on the minimum inhibition concentration (MIC) in a liquid culture, by using Salmonella enteritidis and Bacillus cereus as the test bacteria. The activities of galangin, kaempherol, myricetin and fisetin were each enhanced in the presence of rutin when S. enteritidis was used as the test bacterium. The MIC value for kaempherol was markedly decreased by the addition of rutin. Morin inhibited DNA synthesis, and this effect was promoted by rutin at a concentration of 25 microg/ml.

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